1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor having crystallinity and a method of manufacturing the same.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as a TFT) using a thin film semiconductor is known. This TFT is constructed in such a manner that a thin film semiconductor is formed on a substrate and this thin film semiconductor is used. Although this TFT is used for various kinds of integrated circuits, the TFT receives attention especially as a component of an electrooptical device, especially as a switching element provided in each pixel and a driver element formed in a peripheral circuit portion of an active matrix type liquid crystal display device.
As a thin film semiconductor used for the TFT, although it is easy to use an amorphous silicon film, there is a problem that its electrical characteristics are low. For the purpose of obtaining the improvement in the characteristics of the TFT, it is appropriate that a silicon thin film having crystallinity is used. A silicon film having crystallinity is referred to as polycrystalline silicon, polysilicon, microcrystalline silicon, or the like. In order to obtain the silicon film having the crystallinity, it is appropriate that an amorphous silicon film is first formed, and then, the film is crystallized by heating.
However, in crystallization by heating, it is necessary to carry out heating at a temperature of 600° C. or higher and for 10 hours or more, so that there is a problem that it is difficult to use a glass substrate as a substrate. For example, Corning 7059 glass used for an active type liquid crystal display device has a glass distortion point of 593° C., and there is a problem in heating at a temperature of 600° C. or higher when enlarging the area of a substrate is taken into consideration.
According to the study of the present inventors, it has been found that if heating is carried out after a very small amount of element, such as nickel or palladium, is deposited on the surface of an amorphous silicon film, the amorphous film can be crystallized at a temperature of 550° C. for a processing time of about 4 hours. However, if a large amount of elements as set forth above exist in the semiconductor, the reliability or electrical stability of a device using such semiconductor is damaged, and this is not preferable.
That is, although an element for facilitating crystallization, such as nickel, (in this specification, an element for facilitating crystallization will be referred to as a catalytic element) is necessary when an amorphous silicon film is crystallized, it is desirable not to make the element contained in crystallized silicon to the utmost. Although various methods for gettering the catalytic element in crystallized silicon have been investigated to achieve this object, any method increases the number of steps for gettering and is not very preferable in manufacturing a component.
Although it has been found that when the catalytic element is introduced, crystallization can be made at a temperature of 600° C. or lower for a short time, the crystallinity obtained by the heat treatment has a limit, and there occurs a problem that the crystallinity becomes insufficient.